The technological evolution in the offshore sector tends to an increasing use of high strength steels with yield strength in the range from 80 to 100 ksi for flowlines and risers. In this context, one key component is the riser system, which becomes a more significant factor as water depth increases. Riser system costs are quite sensitive to water depth.
Although in-service conditions and the sensitiveness of environmental loads (i.e. wave and current) are different for the two riser types Top Tension Risers (TTRs) and Steel Catenary Risers (SCRs) for ultra-deep environment, the requirement to reduce raiser weight is extremely important. By reducing the weight of the line, there is a decrease in the cost of the pipe and a significant impact on the tensioning system used to support the riser.
In addition, using high-strength alloy steels can decrease the wall thickness of a pipe up to 30% due to the more efficient design. For riser systems, which rely on buoyancy in the form of aircans for top tension, the thinner wall pipe available with high strength steel allows reduced buoyancy requirements which, in turn, can reduce the hydrodynamic loading on these components and, thus, improve riser response. Riser systems where the tension is reacted by the host facility benefit from high strength steel as the total payload is reduced.
In the past years, there have been several types of high-strength alloy steels developed in the field of quenched and tempered (QT) seamless pipes. These seamless pipes combine both high strength with good toughness and good girth weldability. However, these seamless pipes have wall thickness of up to 40 mm and outside diameter not greater than 22 inches and, thereby, are quite expensive and can only reach a yield strength below 100 ksi after quenching and tempering.
For example, high-strength, weldable steels for seamless pipes have been known in U.S. Pat. No. 6,217,676 which describes an alloy steel that can reach grades of up to X80 after quenching and tempering and has excellent resistance to wet carbon dioxide corrosion and seawater corrosion, comprising in weight % more than 0.10 and 0.30 C, 0.10 to 1.0 Si, 0.1 to 3.0 Mn, 2.5 to less than 7.0 Cr and 0.01 to 0.10 Al, the balance includes Fe and incidental impurities including not more than 0.03% P. However, these types of steels can not reach grades higher than X80 and are quite expensive due to the high content of Cr.
Likewise, U.S. patent application Ser. No. 09/341,722 published Jan. 31, 2002 describes a method for making seamless line pipes within the yield strength range from that of grade X52 to 90 ksi, with a stable elastic limit at high application temperatures by hot-rolling a pipe blank made from a steel which contains 0.06-018% C, Si≦0.40%, 0.80-1.40% Mn, P≦0.025%, S≦0.010%, 0.010-0.060% Al, Mo≦0.50%, Ca≦0.040%, V≦0.10%, Nb≦0.10%, N≦0.015%, and 0.30-1.00% W. However, these types of steels can not reach yield strength higher than 100 ksi and are not weldable in a wide range of heat inputs.
It is, therefore, desirable and advantageous to provide an improved high-strength, weldable alloy steel for seamless pipes to be used in a riser system with yield strength well above 90 ksi and with a wall thickness (WT) to outside diameter (OD) ratio adequate to expected collapse performance which obviates prior art shortcomings and which is able to meet good mechanical properties in the pipe body and weld.